Keita has two older step-sisters, Ako and Riko, but since they aren't related by blood, they love him in a lustful way. After a mishap at school, Ako and Riko finally confess their love to him. Keita dislikes the thought of seeing them other then brother and sister, but as he tries to enter the same school as his sisters, he slowly becomes attracted to them.
KissXSis is one of the most ecchi series I've seen so far but it isn't the kind of ecchi I've seen so far. Usually you'd expect seduction, big breasts and panty shots. KissXSis went beyond that. There was always some kind of tension and it didn't always stay innocent.
The story is original but I felt they could make it a little deeper. I would have liked to see more of the main-characters past and the reason for the current situation.
There were some really funny moments and conversations but also a lot of pointless chatter. You see this quite often but sometimes the characters just said each other's names.
No remarks on the animation. It's pretty recent and you can tell.
KissXSis isn't the most funny series I've seen up until now and sometimes the ecchi was too bizarre for me to handle. If you think you can handle it or if you're into this kind of thing, you should watch it. (a friend of mine really liked it)